Corrosion Behavior of Aluminum in Chloride Mediums

Kivonat:

Aluminum is one of the most widely used materials in corrosive environments due to the effectiveness of the naturally occurring oxide film.
Using different concentrations of Cl solutions it is possible to see some interesting results relating to polarization resistance across several different materials.

Aluminum is one of the most widely used corrosion- resistant metals. The hard and tough oxide film formed on the surface of the aluminum make it resistant to corrosion. If the protective effect of the oxide film is overcome by scratching or by amalgamation, it is attacked under ordinary conditions by hot alkali hydroxides, halogen, various non-metals and even by water.

Corrosion of aluminum has been a subject of numerous studies due to the importance of this material in contemporary civilization. It is well known that there is a potential region in which the rate of corrosion, even in such aggressive media as chlorides. The relatively complex corrosion mechanism of aluminum has been studied by several authors. Corrosion of aluminum occurs only when the metal protective oxide layer is damaged and when the repair mechanism is prevented by chemical dissolution. Polarization methods have been extensively used to investigate the mechanism of localized corrosion and processes that lead to localized corrosion.

By using potentiostatic techniques, the potential is variable. Potentiostatic and potentiodynamic techniques have been applied by several authors to study the corrosion of aluminum in different environment. The exceptional corrosion resistance of aluminum in many environments is due to its protective oxide film which is relatively inert chemically and so provides the passive behavior of aluminum.

In the work of Constantin F. and al. have been studied materials: an AlMn alloy and a sandwich material AlSi/AlMn/AlSi. The aim was to determine the influence of the chloride amount on their corrosion resistance.

These materials are employed in the automotive cooling systems, more specifically for the radiators, and the used liquids they contain which is often a mixture of several salts, such as ammonium or potassium chloride.

The chloride anion is often responsible of the break up of the passive layer spontaneously formed on the Al surface; this passive layer protects efficiently the material.

Three chloride concentrations have been chosen: 100, 300 and 500 ppm. The study was carried out at room temperature with several electrochemical techniques.

Figure 3a represents the evolution of polarization resistance versus time for the AlMn alloy, in Glaceol RX D with different additions of Cl- ions (100, 300 and 500 ppm NaCl). The corrosion experiments were conducted after stabilization of the free corrosion potential. The resistance polarization plots were obtained after immersion in the electrolyte solution (1h, 3h, 9h and 19h), and then the resistance values were calculated by QuickCalc software M352, considering only the linear zone of the curve.

It may be noted that in the case using 100 or 300 ppm NaCl, the polarization resistance increases rapidly in the first half period of the immersion up to 130-165 kOhms.cm2 and after this point (after 9 hours of immersion), it can be observed a fall of approximately four decades: this means that in this period the corrosion rate increases. In 500ppm NaCl medium, it was found that the polarization resistance remains constant throughout the experiment. Their values are a little smaller compared with the values measured for 300 ppm NaCl, at the end of the test (19h).

Figure 3b shows the evolution of polarization resistance versus time for AlSi/AlMn/AlSi alloy, in Glaceol RX D with the same concentrations of Cl- ions. It can be observed that the curve drawn for the media containing 100 ppm has the same appearance with those recorded for AlMn in Glaceol RX D with 100 ppm NaCl. As for the other two additions of NaCl (300 and 500ppm) it can be noticed the same shape of the curve, that remains almost constant in time. The values of the polarization resistance are approximately 100kOhms.cm2 after the test period.

Figure 3: Evolution of polarization resistance versus time in Glaceol RX D with different additions of Clions: a. AlMn and b. AlSi/AlMn/AlSi

Figure 4 shows the evolution of the resistance polarization versus immersion time for the two materials in Glaceol RX D (with 100 and 500 ppm NaCl). The effect of AlSi can be observed in comparison with AlMn uncovered. In both solutions the corrosion rate is lower than in the case of the aluminum sandwich material; the value of the resistance polarization is almost 200 kOhms.cm2, for the solution containing 100 ppm NaCl, respectively 140 kOhms.cm2 for that containing 500 ppm NaCl.

Figure 4: Evolution of polarization resistance versus time in Glaceol RX D - comparison of the two aluminium alloys: a. with 100 ppm NaCl and b. with 500 ppm NaCl

Conclusions
The two investigated materials, AlMn and AlSi/AlMn/AlSi alloys, have shown good enough corrosion behaviors in the cooling liquid, Glaceol RX D (30vol.%) recommended by Dacia constructor, even with addition of Cl- ions (100 to 500 ppm NaCl). These additions represent the eventuality of Glaceol Dilution in natural distributive water and lead to more corrosive media. It has been shown, in this work, the importance of the Cl- amount, hence the importance of using unmineralized water to dilute the antifreeze liquid. Better behavior of the AlSi/AlMn/AlSi then AlMn was also observed in all covered media.

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